The concentration and molecular variants of alpha-fetoprotein present in serum and amniotic fluid of fetal, newborn and pregnant rats is being measured. Two classes of molecular variants will be examined. Two alpha-fetoprotein molecular variants are found when rat serum is chromatographed on an agarose-concanavalin A affinity matrix. In addition, two different molecular variants are observed when rat serum is studied by acrylamide gel electrophoresis. Methods have been developed by which the concentrations of these variants and their ratios can be determined and by which variants can be prepared on a preparative scale. The chemicl and physical properties of these molecular variants of alphafetoprotein will be determined using standard methods of protein characterization.